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Simple argument question

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  Simple argument question Bint <bint@ign.com> - 2011-12-16 18:46 -0600
    Re: Simple argument question "Paul" <pchrist<nospam>or@yahoo.co.uk> - 2011-12-17 01:13 +0000
      Re: Simple argument question Bint <bint@ign.com> - 2011-12-17 12:06 -0600
        Re: Simple argument question "Paul" <pchrist<nospam>or@yahoo.co.uk> - 2011-12-17 18:45 +0000
          Re: Simple argument question "Paul" <pchrist<nospam>or@yahoo.co.uk> - 2011-12-17 23:16 +0000
            Re: Simple argument question jt@toerring.de (Jens Thoms Toerring) - 2011-12-18 22:07 +0000
              Re: Simple argument question "Paul" <pchrist<nospam>or@yahoo.co.uk> - 2011-12-18 22:34 +0000
    Re: Simple argument question 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-17 00:13 -0800
      Re: Simple argument question Juha Nieminen <nospam@thanks.invalid> - 2011-12-17 08:16 +0000
        Re: Simple argument question 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-17 00:51 -0800
          Re: Simple argument question Juha Nieminen <nospam@thanks.invalid> - 2011-12-17 10:23 +0000
    Re: Simple argument question Goran <goran.pusic@gmail.com> - 2011-12-17 23:08 -0800
      Re: Simple argument question Heck <heckja_in@ordertostymieharvestersverizon.net> - 2012-01-05 09:59 -0500
    Re: Simple argument question Krice <paulkp@mbnet.fi> - 2011-12-18 00:46 -0800
      Re: Simple argument question jt@toerring.de (Jens Thoms Toerring) - 2011-12-18 22:14 +0000
        Re: Simple argument question Jorgen Grahn <grahn+nntp@snipabacken.se> - 2011-12-19 12:01 +0000
          Re: Simple argument question Tobias Müller <troplin@bluewin.ch> - 2011-12-19 17:17 +0000
            Re: Simple argument question Jorgen Grahn <grahn+nntp@snipabacken.se> - 2011-12-20 00:33 +0000
    Re: Simple argument question Jorgen Grahn <grahn+nntp@snipabacken.se> - 2011-12-18 17:03 +0000
      Re: Simple argument question 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-18 10:04 -0800

#12603 — Simple argument question

Hi can someone answer a simple question involving arguments to functions?

I need to call a library function which expects an argument like this
Function(double pointlist[][3]);

My question is how do I make one of those if I am just wanting to malloc the
memory?  Can I use a double pointer?

Can I have something like this:
double *points = (double *)malloc(10*3*sizeof(double));

for int (j = 0; j < 10; j++) {
     *(points + j*3 + 0) = x;
     *(points + j*3 + 1) = y;
     *(points + j*3 + 2) = z;
}

And then call the function with points as the argument?  It doesn't compile
but I was wondering if I could cast it?  Or do I need something else instead
of a double pointer for my points?

Does that make sense? I am confused about how to call the function with
anything but something like:
 double array[500][3];

I don't want that because I don't know the number of points, see?

Thanks for any help!
B

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#12604

"Bint" <bint@ign.com> wrote in message news:CB114269.B5F7%bint@ign.com...
> Hi can someone answer a simple question involving arguments to functions?
>
> I need to call a library function which expects an argument like this
> Function(double pointlist[][3]);
>
> My question is how do I make one of those if I am just wanting to malloc 
> the
> memory?  Can I use a double pointer?
>
> Can I have something like this:
> double *points = (double *)malloc(10*3*sizeof(double));
>
> for int (j = 0; j < 10; j++) {
>     *(points + j*3 + 0) = x;
>     *(points + j*3 + 1) = y;
>     *(points + j*3 + 2) = z;
> }
>
> And then call the function with points as the argument?  It doesn't 
> compile
> but I was wondering if I could cast it?  Or do I need something else 
> instead
> of a double pointer for my points?
>
> Does that make sense? I am confused about how to call the function with
> anything but something like:
> double array[500][3];
>
> I don't want that because I don't know the number of points, see?
>
Hi.
When you create 2d arrays using new you may use pointer to pointer types , 
but this is a different type from the built in array type.
In your function argument your 2d array will decay into a pointer of type:
double (*)[3]

Each dimensional array is pointed to by a pointer of type one dimension 
less. For example a one dimensional array of doubles is pointed to by a 
double*( the same as a pointer to a single double ). A 2-d array is pointed 
to by a type T(*)[S] . a 3d array is pointed to by a type T(*)[S1][S2] etc 
etc.

When you create your pointer with malloc you  are creating a pointer of type 
double* which is a pointer to a 1d array, or a pointer to a single double. 
But the function argument needs you to pass a 2d array.










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#12623

> "Bint" <bint@ign.com> wrote in message news:CB114269.B5F7%bint@ign.com...
>> Hi can someone answer a simple question involving arguments to functions?
>> 
>> I need to call a library function which expects an argument like this
>> Function(double pointlist[][3]);
>> 
>> My question is how do I make one of those if I am just wanting to malloc
>> the
>> memory?  Can I use a double pointer?
>> 
>> Can I have something like this:
>> double *points = (double *)malloc(10*3*sizeof(double));
>> 
>> for int (j = 0; j < 10; j++) {
>>     *(points + j*3 + 0) = x;
>>     *(points + j*3 + 1) = y;
>>     *(points + j*3 + 2) = z;
>> }
>> 
>> And then call the function with points as the argument?  It doesn't
>> compile
>> but I was wondering if I could cast it?  Or do I need something else
>> instead
>> of a double pointer for my points?
>> 
>> Does that make sense? I am confused about how to call the function with
>> anything but something like:
>> double array[500][3];
>> 
>> I don't want that because I don't know the number of points, see?
>> 
> Hi.
> When you create 2d arrays using new you may use pointer to pointer types ,
> but this is a different type from the built in array type.
> In your function argument your 2d array will decay into a pointer of type:
> double (*)[3]
> 
> Each dimensional array is pointed to by a pointer of type one dimension
> less. For example a one dimensional array of doubles is pointed to by a
> double*( the same as a pointer to a single double ). A 2-d array is pointed
> to by a type T(*)[S] . a 3d array is pointed to by a type T(*)[S1][S2] etc
> etc.
> 
> When you create your pointer with malloc you  are creating a pointer of type
> double* which is a pointer to a 1d array, or a pointer to a single double.
> But the function argument needs you to pass a 2d array.
> 

So how would I create and pass something valid to this function, say for an
array of [10][3], but specifying that 10 at run-time?  Do I use new instead
of malloc?

Thanks
B

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#12624

"Bint" <bint@ign.com> wrote in message news:CB123632.B739%bint@ign.com...
>
>> "Bint" <bint@ign.com> wrote in message news:CB114269.B5F7%bint@ign.com...
>>> Hi can someone answer a simple question involving arguments to 
>>> functions?
>>>
>>> I need to call a library function which expects an argument like this
>>> Function(double pointlist[][3]);
>>>
>>> My question is how do I make one of those if I am just wanting to malloc
>>> the
>>> memory?  Can I use a double pointer?
>>>
>>> Can I have something like this:
>>> double *points = (double *)malloc(10*3*sizeof(double));
>>>
>>> for int (j = 0; j < 10; j++) {
>>>     *(points + j*3 + 0) = x;
>>>     *(points + j*3 + 1) = y;
>>>     *(points + j*3 + 2) = z;
>>> }
>>>
>>> And then call the function with points as the argument?  It doesn't
>>> compile
>>> but I was wondering if I could cast it?  Or do I need something else
>>> instead
>>> of a double pointer for my points?
>>>
>>> Does that make sense? I am confused about how to call the function with
>>> anything but something like:
>>> double array[500][3];
>>>
>>> I don't want that because I don't know the number of points, see?
>>>
>> Hi.
>> When you create 2d arrays using new you may use pointer to pointer types 
>> ,
>> but this is a different type from the built in array type.
>> In your function argument your 2d array will decay into a pointer of 
>> type:
>> double (*)[3]
>>
>> Each dimensional array is pointed to by a pointer of type one dimension
>> less. For example a one dimensional array of doubles is pointed to by a
>> double*( the same as a pointer to a single double ). A 2-d array is 
>> pointed
>> to by a type T(*)[S] . a 3d array is pointed to by a type T(*)[S1][S2] 
>> etc
>> etc.
>>
>> When you create your pointer with malloc you  are creating a pointer of 
>> type
>> double* which is a pointer to a 1d array, or a pointer to a single 
>> double.
>> But the function argument needs you to pass a 2d array.
>>
>
> So how would I create and pass something valid to this function, say for 
> an
> array of [10][3], but specifying that 10 at run-time?  Do I use new 
> instead
> of malloc?
>
To solve your problem you need to cast the return value from the allocation 
function to a suitable pointer type, which is compatable with the function 
argument.
IIRC malloc returns a void pointer so intead of casting it to a double*, as 
you have done in your example, you can cast it to a double(*)[3]. This 
pointer type would then be suitable to pass to the function:

double (*points)[3] = (double (*)[3])malloc(10*3*sizeof(double));

Function( points);


HTH. 



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#12629

"Paul or@yahoo.co.uk>" <pchrist<nospam> wrote in message 
news:jcio0k$1tni$1@adenine.netfront.net...
>
> "Bint" <bint@ign.com> wrote in message news:CB123632.B739%bint@ign.com...
>>
>>> "Bint" <bint@ign.com> wrote in message 
>>> news:CB114269.B5F7%bint@ign.com...
>>>> Hi can someone answer a simple question involving arguments to 
>>>> functions?
>>>>
>>>> I need to call a library function which expects an argument like this
>>>> Function(double pointlist[][3]);
>>>>
>>>> My question is how do I make one of those if I am just wanting to 
>>>> malloc
>>>> the
>>>> memory?  Can I use a double pointer?
>>>>
>>>> Can I have something like this:
>>>> double *points = (double *)malloc(10*3*sizeof(double));
>>>>
>>>> for int (j = 0; j < 10; j++) {
>>>>     *(points + j*3 + 0) = x;
>>>>     *(points + j*3 + 1) = y;
>>>>     *(points + j*3 + 2) = z;
>>>> }
>>>>
>>>> And then call the function with points as the argument?  It doesn't
>>>> compile
>>>> but I was wondering if I could cast it?  Or do I need something else
>>>> instead
>>>> of a double pointer for my points?
>>>>
>>>> Does that make sense? I am confused about how to call the function with
>>>> anything but something like:
>>>> double array[500][3];
>>>>
>>>> I don't want that because I don't know the number of points, see?
>>>>
>>> Hi.
>>> When you create 2d arrays using new you may use pointer to pointer types 
>>> ,
>>> but this is a different type from the built in array type.
>>> In your function argument your 2d array will decay into a pointer of 
>>> type:
>>> double (*)[3]
>>>
>>> Each dimensional array is pointed to by a pointer of type one dimension
>>> less. For example a one dimensional array of doubles is pointed to by a
>>> double*( the same as a pointer to a single double ). A 2-d array is 
>>> pointed
>>> to by a type T(*)[S] . a 3d array is pointed to by a type T(*)[S1][S2] 
>>> etc
>>> etc.
>>>
>>> When you create your pointer with malloc you  are creating a pointer of 
>>> type
>>> double* which is a pointer to a 1d array, or a pointer to a single 
>>> double.
>>> But the function argument needs you to pass a 2d array.
>>>
>>
>> So how would I create and pass something valid to this function, say for 
>> an
>> array of [10][3], but specifying that 10 at run-time?  Do I use new 
>> instead
>> of malloc?
>>
> To solve your problem you need to cast the return value from the 
> allocation function to a suitable pointer type, which is compatable with 
> the function argument.
> IIRC malloc returns a void pointer so intead of casting it to a double*, 
> as you have done in your example, you can cast it to a double(*)[3]. This 
> pointer type would then be suitable to pass to the function:
>
> double (*points)[3] = (double (*)[3])malloc(10*3*sizeof(double));
>
> Function( points);
>
>
> HTH.
>
Note: your for loop will need to be modified as each increment on this type 
of pointer will increment it by a complete row of 3 elelments. 



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#12643

"Paul" <pchrist <nospam>or@yahoo.co.uk> wrote:

> "Paul or@yahoo.co.uk>" <pchrist<nospam> wrote in message 
> news:jcio0k$1tni$1@adenine.netfront.net...
> >
> > "Bint" <bint@ign.com> wrote in message news:CB123632.B739%bint@ign.com...
> >>
> >>> "Bint" <bint@ign.com> wrote in message 
> >>> news:CB114269.B5F7%bint@ign.com...
> >>>> Hi can someone answer a simple question involving arguments to 
> >>>> functions?
> >>>>
> >>>> I need to call a library function which expects an argument like this
> >>>> Function(double pointlist[][3]);
> >>>>
> >>>> My question is how do I make one of those if I am just wanting to 
> >>>> malloc
> >>>> the
> >>>> memory?  Can I use a double pointer?
> >>>>
> >>>> Can I have something like this:
> >>>> double *points = (double *)malloc(10*3*sizeof(double));
> >>>>
> >>>> for int (j = 0; j < 10; j++) {
> >>>>     *(points + j*3 + 0) = x;
> >>>>     *(points + j*3 + 1) = y;
> >>>>     *(points + j*3 + 2) = z;
> >>>> }
> >>>>
> >>>> And then call the function with points as the argument?  It doesn't
> >>>> compile
> >>>> but I was wondering if I could cast it?  Or do I need something else
> >>>> instead
> >>>> of a double pointer for my points?
> >>>>
> >>>> Does that make sense? I am confused about how to call the function with
> >>>> anything but something like:
> >>>> double array[500][3];
> >>>>
> >>>> I don't want that because I don't know the number of points, see?
> >>>>
> >>> Hi.
> >>> When you create 2d arrays using new you may use pointer to pointer types 
> >>> ,
> >>> but this is a different type from the built in array type.
> >>> In your function argument your 2d array will decay into a pointer of 
> >>> type:
> >>> double (*)[3]
> >>>
> >>> Each dimensional array is pointed to by a pointer of type one dimension
> >>> less. For example a one dimensional array of doubles is pointed to by a
> >>> double*( the same as a pointer to a single double ). A 2-d array is 
> >>> pointed
> >>> to by a type T(*)[S] . a 3d array is pointed to by a type T(*)[S1][S2] 
> >>> etc
> >>> etc.
> >>>
> >>> When you create your pointer with malloc you  are creating a pointer of 
> >>> type
> >>> double* which is a pointer to a 1d array, or a pointer to a single 
> >>> double.
> >>> But the function argument needs you to pass a 2d array.
> >>>
> >>
> >> So how would I create and pass something valid to this function, say for 
> >> an
> >> array of [10][3], but specifying that 10 at run-time?  Do I use new 
> >> instead
> >> of malloc?
> >>
> > To solve your problem you need to cast the return value from the 
> > allocation function to a suitable pointer type, which is compatable with 
> > the function argument.
> > IIRC malloc returns a void pointer so intead of casting it to a double*, 
> > as you have done in your example, you can cast it to a double(*)[3]. This 
> > pointer type would then be suitable to pass to the function:
> >
> > double (*points)[3] = (double (*)[3])malloc(10*3*sizeof(double));
> >
> > Function( points);
> >
> >
> > HTH.
> >
> Note: your for loop will need to be modified as each increment on this type 
> of pointer will increment it by a complete row of 3 elelments. 

I guess you mean that he should remove the multiplication of
'j' by three. But then, as an added bonus due to getting the
types right, he now could rewrite the initialization loop more
naturally as

for ( size_t j = 0; j < 10; ++j ) {
    points[ j ][ 0 ] = x;
	points[ j ][ 1 ] = y;
	points[ j ][ 2 ] = z;
}

What I am still wondering about though is how the function re-
ceiving the 2-d array is supposed to know how many "points"
it will get - all it receives is a pointer to (the first three
coordinates of) the first point, but no information about how
many more there are...
                                     Regards, Jens
-- 
  \   Jens Thoms Toerring  ___      jt@toerring.de
   \__________________________      http://toerring.de

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#12645

"Jens Thoms Toerring" <jt@toerring.de> wrote in message 
news:9l76heFm6gU1@mid.uni-berlin.de...
> "Paul" <pchrist <nospam>or@yahoo.co.uk> wrote:
>
>> "Paul or@yahoo.co.uk>" <pchrist<nospam> wrote in message
>> news:jcio0k$1tni$1@adenine.netfront.net...
>> >
>> > "Bint" <bint@ign.com> wrote in message 
>> > news:CB123632.B739%bint@ign.com...
>> >>
>> >>> "Bint" <bint@ign.com> wrote in message
>> >>> news:CB114269.B5F7%bint@ign.com...
>> >>>> Hi can someone answer a simple question involving arguments to
>> >>>> functions?
>> >>>>
>> >>>> I need to call a library function which expects an argument like 
>> >>>> this
>> >>>> Function(double pointlist[][3]);
>> >>>>
>> >>>> My question is how do I make one of those if I am just wanting to
>> >>>> malloc
>> >>>> the
>> >>>> memory?  Can I use a double pointer?
>> >>>>
>> >>>> Can I have something like this:
>> >>>> double *points = (double *)malloc(10*3*sizeof(double));
>> >>>>
>> >>>> for int (j = 0; j < 10; j++) {
>> >>>>     *(points + j*3 + 0) = x;
>> >>>>     *(points + j*3 + 1) = y;
>> >>>>     *(points + j*3 + 2) = z;
>> >>>> }
>> >>>>
>> >>>> And then call the function with points as the argument?  It doesn't
>> >>>> compile
>> >>>> but I was wondering if I could cast it?  Or do I need something else
>> >>>> instead
>> >>>> of a double pointer for my points?
>> >>>>
>> >>>> Does that make sense? I am confused about how to call the function 
>> >>>> with
>> >>>> anything but something like:
>> >>>> double array[500][3];
>> >>>>
>> >>>> I don't want that because I don't know the number of points, see?
>> >>>>
>> >>> Hi.
>> >>> When you create 2d arrays using new you may use pointer to pointer 
>> >>> types
>> >>> ,
>> >>> but this is a different type from the built in array type.
>> >>> In your function argument your 2d array will decay into a pointer of
>> >>> type:
>> >>> double (*)[3]
>> >>>
>> >>> Each dimensional array is pointed to by a pointer of type one 
>> >>> dimension
>> >>> less. For example a one dimensional array of doubles is pointed to by 
>> >>> a
>> >>> double*( the same as a pointer to a single double ). A 2-d array is
>> >>> pointed
>> >>> to by a type T(*)[S] . a 3d array is pointed to by a type 
>> >>> T(*)[S1][S2]
>> >>> etc
>> >>> etc.
>> >>>
>> >>> When you create your pointer with malloc you  are creating a pointer 
>> >>> of
>> >>> type
>> >>> double* which is a pointer to a 1d array, or a pointer to a single
>> >>> double.
>> >>> But the function argument needs you to pass a 2d array.
>> >>>
>> >>
>> >> So how would I create and pass something valid to this function, say 
>> >> for
>> >> an
>> >> array of [10][3], but specifying that 10 at run-time?  Do I use new
>> >> instead
>> >> of malloc?
>> >>
>> > To solve your problem you need to cast the return value from the
>> > allocation function to a suitable pointer type, which is compatable 
>> > with
>> > the function argument.
>> > IIRC malloc returns a void pointer so intead of casting it to a 
>> > double*,
>> > as you have done in your example, you can cast it to a double(*)[3]. 
>> > This
>> > pointer type would then be suitable to pass to the function:
>> >
>> > double (*points)[3] = (double (*)[3])malloc(10*3*sizeof(double));
>> >
>> > Function( points);
>> >
>> >
>> > HTH.
>> >
>> Note: your for loop will need to be modified as each increment on this 
>> type
>> of pointer will increment it by a complete row of 3 elelments.
>
> I guess you mean that he should remove the multiplication of
> 'j' by three. But then, as an added bonus due to getting the
> types right, he now could rewrite the initialization loop more
> naturally as
>
> for ( size_t j = 0; j < 10; ++j ) {
>    points[ j ][ 0 ] = x;
> points[ j ][ 1 ] = y;
> points[ j ][ 2 ] = z;
> }
Yes I was thinking of suggesting this to the OP. It's much nicer this way.

>
> What I am still wondering about though is how the function re-
> ceiving the 2-d array is supposed to know how many "points"
> it will get - all it receives is a pointer to (the first three
> coordinates of) the first point, but no information about how
> many more there are...

This was also what I  thought. I guess the OP has simplified the function 
and the 'actual' lib function will also have a size parameter. Or maybe the 
function has access to the runtime size parameter by some other channel, 
IDK. 



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#12613

On Saturday, December 17, 2011 8:46:01 AM UTC+8, Bint wrote:
> Hi can someone answer a simple question involving arguments to functions?
> 
> I need to call a library function which expects an argument like this
> Function(double pointlist[][3]);
> 
> My question is how do I make one of those if I am just wanting to malloc the
> memory?  Can I use a double pointer?
> 
> Can I have something like this:
> double *points = (double *)malloc(10*3*sizeof(double));
> 
> for int (j = 0; j < 10; j++) {
>      *(points + j*3 + 0) = x;
>      *(points + j*3 + 1) = y;
>      *(points + j*3 + 2) = z;
> }
> 
> And then call the function with points as the argument?  It doesn't compile
> but I was wondering if I could cast it?  Or do I need something else instead
> of a double pointer for my points?
> 
> Does that make sense? I am confused about how to call the function with
> anything but something like:
>  double array[500][3];
> 
> I don't want that because I don't know the number of points, see?
> 
> Thanks for any help!
> B

The size of  a double  is platform dependent. 

Is the compiler smart enough to figure out the  correct value of 
points + j*3 ? 

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#12614

88888 Dihedral <dihedral88888@googlemail.com> wrote:
> Is the compiler smart enough to figure out the  correct value of 
> points + j*3 ? 

  If it weren't, then the compiler would be non-standard.

  What do you think "points + j*3" means?

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#12616

On Saturday, December 17, 2011 4:16:13 PM UTC+8, Juha Nieminen wrote:
> 88888 Dihedral <dihedr...@googlemail.com> wrote:
> > Is the compiler smart enough to figure out the  correct value of 
> > points + j*3 ? 
> 
>   If it weren't, then the compiler would be non-standard.
> 
>   What do you think "points + j*3" means?

points+1 means  add to ponits 1 size of a double but if a double has to start at some limited address, e.g. 8 byte boundaries only , some strange thing might happen if the compiler is not smart enough

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#12618

88888 Dihedral <dihedral88888@googlemail.com> wrote:
> On Saturday, December 17, 2011 4:16:13 PM UTC+8, Juha Nieminen wrote:
>> 88888 Dihedral <dihedr...@googlemail.com> wrote:
>> > Is the compiler smart enough to figure out the  correct value of 
>> > points + j*3 ? 
>> 
>>   If it weren't, then the compiler would be non-standard.
>> 
>>   What do you think "points + j*3" means?
> 
> points+1 means  add to ponits 1 size of a double but if a double has to start at some limited address, e.g. 8 byte boundaries only , some strange thing might happen if the compiler is not smart enough

  If "points" is a valid pointer to an array (of at least 2 elements), then
why wouldn't "points + 1" be a valid pointer?

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#12635

On Dec 17, 1:46 am, Bint <b...@ign.com> wrote:
> Hi can someone answer a simple question involving arguments to functions?
>
> I need to call a library function which expects an argument like this
> Function(double pointlist[][3]);
>
> My question is how do I make one of those if I am just wanting to malloc the
> memory?  Can I use a double pointer?
>
> Can I have something like this:
> double *points = (double *)malloc(10*3*sizeof(double));
>
> for int (j = 0; j < 10; j++) {
>      *(points + j*3 + 0) = x;
>      *(points + j*3 + 1) = y;
>      *(points + j*3 + 2) = z;
>
> }
>
> And then call the function with points as the argument?  It doesn't compile
> but I was wondering if I could cast it?  Or do I need something else instead
> of a double pointer for my points?
>
> Does that make sense? I am confused about how to call the function with
> anything but something like:
>  double array[500][3];
>
> I don't want that because I don't know the number of points, see?

Problem decomposition to the rescue! ;-)

Here's a possibly better way to look at things: double pointlist [][3]
says that "Function" receives an array of double[3] (do you know of
cdecl.org? Try it, it helps with such things). So let's call that D3:

typedef double D3[3];

(More precisely, Function receives a pointer to a D3 (D3* pointlist),
but one can write that as double pointlist[][3], or D3 pointlist[];
that is, all of

void Function(double pointlist[][3]);
void Function(D3 pointlist[]);
void Function(D3* pointlist);

are one and the same as far as compiler is concerned. IMO, the third
form is best because it conveys the meaning the best, and avoids much
of mental juggling about the "feature" of C in play here, the array
decaying to a pointer when appearing in an expression.)

Now... If you decide to go for C++ route, things get easier on the
call side:

void Function(D3* pointlist) {...}

D3* p = new D3[10];
Function(p);

If you still want pure C, things are again a bit easier (IMO):

D3* p = malloc(10*sizeof(D3));
Function(p);

Finally, as others noted, "points + j*3 + 0" is wrong. You need
points[j][0]. Again, this is because "points" is a pointer to a D3,
and when you increment it, you get a pointer to the next D3 (not a
pointer to a double). And, of course, when you dereference that, you
get a D3, not double, so about that casting: no, that's the wrong
idea. Further, IMO, in pointer manipulation such as the above, a cast
indicates a bug.

Goran.

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#13122

I would have answered, use 
double **points = (double **)malloc(10*3*sizeof(double));

, to illustrate the principles involved, but I know better.

Building a type up with successive typedefs is also an old C technique
and here it illustrates the principles, too, no quarrel.


On Sat, 17 Dec 2011 23:08:30 -0800 (PST), Goran
<goran.pusic@gmail.com> wrote:
>Problem decomposition to the rescue! ;-)
>
>typedef double D3[3];

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#12636

On 17 joulu, 02:46, Bint <b...@ign.com> wrote:
> And then call the function with points as the argument?  It doesn't compile

Let's rewrite this to C++:

1. Create a point class or struct with x, y, z variables
2. Use std::vector to store them if you don't know the size of the
array
3. Pass vector's reference to a function processing the array

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#12644

Krice <paulkp@mbnet.fi> wrote:
> On 17 joulu, 02:46, Bint <b...@ign.com> wrote:
> > And then call the function with points as the argument?  It doesn't compile

> Let's rewrite this to C++:

> 1. Create a point class or struct with x, y, z variables
> 2. Use std::vector to store them if you don't know the size of the
> array
> 3. Pass vector's reference to a function processing the array

Fine as long as the function receiving the array of points isn't
some external function from a library written in C that can't be
simply made to work with C++ classes and vectors...

                               Regards, Jens
-- 
  \   Jens Thoms Toerring  ___      jt@toerring.de
   \__________________________      http://toerring.de

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#12657

On Sun, 2011-12-18, Jens Thoms Toerring wrote:
> Krice <paulkp@mbnet.fi> wrote:
>> On 17 joulu, 02:46, Bint <b...@ign.com> wrote:
>> > And then call the function with points as the argument?  It doesn't compile
>
>> Let's rewrite this to C++:
>
>> 1. Create a point class or struct with x, y, z variables
>> 2. Use std::vector to store them if you don't know the size of the
>> array
>> 3. Pass vector's reference to a function processing the array
>
> Fine as long as the function receiving the array of points isn't
> some external function from a library written in C that can't be
> simply made to work with C++ classes and vectors...

Surely we cannot assume that is the case? He /did/ post to
comp.lang.c++ and said nothing about such limitations as far as I can
tell.

/Jorgen

-- 
  // Jorgen Grahn <grahn@  Oo  o.   .     .
\X/     snipabacken.se>   O  o   .

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#12664

Jorgen Grahn <grahn+nntp@snipabacken.se> wrote:

> Surely we cannot assume that is the case? He /did/ post to
> comp.lang.c++ and said nothing about such limitations as far as I can
> tell.
> 
> /Jorgen

He /did/ say that he wants to call a library function. Whether that library
is in C or C++ is not important, since the signature is valid C++.

Normally if someone speaks of library function, he means a function that he
cannot modify himself. And even if he could, he asked how to /call/ that
function not how to turn the function into beautiful C++.

Of course it is not bad to give advice how to write beautiful C++ code, but
you cannot say that the question was not detailed enough.

Tobi

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#12670

On Mon, 2011-12-19, Tobias Müller wrote:
> Jorgen Grahn <grahn+nntp@snipabacken.se> wrote:
>
>> Surely we cannot assume that is the case? He /did/ post to
>> comp.lang.c++ and said nothing about such limitations as far as I can
>> tell.
>> 
>> /Jorgen
>
> He /did/ say that he wants to call a library function. Whether that library
> is in C or C++ is not important, since the signature is valid C++.

Oops, my mistake. He did write:
>>> I need to call a library function which expects an argument like this
>>> Function(double pointlist[][3]);

It's an unfortunate choice of interface even for a C function IMHO,
but it's probably not something he has control over.

/Jorgen

-- 
  // Jorgen Grahn <grahn@  Oo  o.   .     .
\X/     snipabacken.se>   O  o   .

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#12637

On Sat, 2011-12-17, Bint wrote:
> Hi can someone answer a simple question involving arguments to functions?
>
> I need to call a library function which expects an argument like this
> Function(double pointlist[][3]);
>
> My question is how do I make one of those if I am just wanting to malloc the
> memory?  Can I use a double pointer?
...

Switch to a datatype which uses std::vector, or perhaps vectors of
vectors.  Much easier and safer to handle.

/Jorgen

-- 
  // Jorgen Grahn <grahn@  Oo  o.   .     .
\X/     snipabacken.se>   O  o   .

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#12642

On Monday, December 19, 2011 1:03:09 AM UTC+8, Jorgen Grahn wrote:
> On Sat, 2011-12-17, Bint wrote:
> > Hi can someone answer a simple question involving arguments to functions?
> >
> > I need to call a library function which expects an argument like this
> > Function(double pointlist[][3]);
> >
> > My question is how do I make one of those if I am just wanting to malloc the
> > memory?  Can I use a double pointer?
> ...
> 
> Switch to a datatype which uses std::vector, or perhaps vectors of
> vectors.  Much easier and safer to handle.
> 
> /Jorgen

But that means slower most of the time. There's no free lunch. 
I prefer to re-size my own pointer manually at certain situations only 
but this not for a novice. 

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